Energetics of syntrophic fatty acid oxidation

Fatty acids are key intermediates in methanogenic degradation of organic matter in sediments as well as in anaerobic reactors. Conversion of butyrate or propionate to acetate, (CO2), and hydrogen is endergonic under standard conditions, and becomes possible only at low hydrogen concentrations (10 4--10-5 bar). A model of energy sharing between fermenting and methanogenic bacteria attributes a maximum amount of about 20 kJ per mol reaction to each partner in this syntrophic cooperation system. This amount corresponds to synthesis of only a fraction (one-third) of an ATP to be synthesized per reaction. Recent studies on the biochemistry of syntrophic fatty acid-oxidizing bacteria have revealed that hydrogen release from butyrate by these bacteria is inhibited by a protonophore or the ATPase inhibitor DCCD (N,N'-dicyclohexyl carbodiimide), indicating that a reversed electron transport step is involved in butyrate or propionate oxidation. Hydrogenase, butyryl-CoA dehydrogenase, and succinate dehydrogenase acitivities were found to be partially associated with the cytoplasmic membrane fraction. Also glycolic acid is degraded to methane and CO 2 by a defined syntrophic coculture. Here the most difficult step for hydrogen release is the glycolate dehydrogenase reaction ( E ~ = 9 2 mV). Glycolate dehydrogenase, hydrogenase, and ATPase were found to be membrane-bound enzymes. Membrane vesicles produced hydrogen from glycolate only in the presence of ATP; protonophores and DCCD inhibited this hydrogen release. This system provides a suitable model to study reversed electron transport in interspecies hydrogen transfer between fermenting and methanogenic bacteria in methanogenic biomass degradation.